1. Field of the Invention
The present invention relates to a feed control method for consumable electrode AC arc welding to obtain a desired bead shape.
2. Description of the Related Art
FIG. 5 is a current-voltage waveform diagram of consumable electrode AC arc welding. In the figure, (A) is a polarity switching signal Spn, (B) is the welding current Iw, (C) is the welding voltage Vw, and (D) is the welding wire feed rate Fw (cm/minute). The figure is for a case of short circuiting transfer arc welding, but substantially similar results are obtained for globular transfer welding and spray transfer welding. In the following explanation, the welding current Iw, welding voltage Vw, and output voltage E explained below refer to absolute values. Hence a statement “a value is large” means that the absolute value is large.
As shown in (A) in the figure, in consumable electrode AC arc welding, welding is performed while switching in alternation between an electrode positive polarity period Tep and an electrode negative polarity period Ten, according to a polarity switching signal Spn. At this time, as indicated in (D) in the figure, the feed rate Fw is constant, independent of the polarity.
As shown in (A) in the figure, at time t1, the polarity switching signal Spn goes to high level, switching to the electrode positive polarity EP, upon which, as shown in (B) in the figure, the welding current Iw for electrode positive polarity EP flows, and as shown in (C) in the figure, the welding voltage Vw of electrode positive polarity EP is applied between the welding wire and the base material. During the short circuit period Ts between times t1 and t2, as shown in (B) in the figure, the welding current Iw gradually increases, and as shown in (C) in the figure, the welding voltage Vw reaches a low short circuit voltage value of approximately several volts. During the arc period Ta between times t2 and t3, as shown in (B) in the figure, the welding current Iw gradually decreases, and as shown in (C) in the figure, the welding voltage Vw reaches the arc voltage value of approximately several tens of volts. Thereafter, until the electrode positive polarity period Tep ends at time t4, there is repetition of the short circuit period Ts and arc period Ta.
As shown in (A) in the figure, at time t4, the polarity switching signal Spn goes to low level, and when the welding power supply output switches to electrode negative polarity EN, an electrode negative polarity EN welding current Iw flows as shown in (B), and as shown in (C), the welding voltage Vw is applied. The short circuit period Ts between times t4 and t5 and the arc period Ta between times t5 and t6 are repeated as above until the end of the electrode negative polarity period Ten at time t7.
FIG. 6 shows an example of wire welding characteristics for electrode positive polarity EP and for electrode negative polarity EN. In the figure, the horizontal axis indicates the welding current average value for each polarity, and the vertical axis indicates the feed rate Fw. The figure shows results for MAG welding using an iron wire of diameter 1.2 mm. The following explanation references this figure.
As shown in the figure, the wire welding characteristics differ greatly for electrode positive polarity EP and for electrode negative polarity EN. As stated above, in consumable electrode AC arc welding, the feed rate Fw is a constant value independent of polarity, so that if for example the feed rate is set to Fw=350 cm/minute, the welding current average value for electrode positive polarity EP is Iep=140 A, and the welding current average value for electrode negative polarity EN is Ien=100 A. In FIG. 5, the average of the welding current value during the electrode positive polarity period Tep is the electrode positive polarity welding current average value Iep, and the average of the welding current value during the electrode negative polarity period Ten is the electrode negative polarity welding current average value Ien. In consumable electrode AC arc welding, the EN ratio Ren (%)=100×(Ten/(Tep+Ten)), which is the ratio of the above electrode positive polarity period Tep to the electrode negative polarity period Ten, may be varied to change the wire welding characteristics, so that a desired bead shape (penetration depth, reinforcement height, and similar) is obtained (see for example Japanese Patent Publication S62-55952, Japanese Patent Laid-open 2004-114088).
The welding current average value Iav over all periods (Tep+Ten) can be represented by the following equation, taking as parameters the electrode positive polarity welding current average value Iep, electrode negative polarity welding current average value Ien, and the EN ratio Ren.Iav=(Ien−Iep)×(Ren/100)+Iep  (1)
The numerical example explained above with reference to FIG. 6 is substituted into equation (1). That is, upon substituting Iep=140 A and Ien=100 A, the following is obtained.Iav=−0.4×Ren+100
This relation is plotted in the graph shown in FIG. 7. In the figure, the horizontal axis represents the EN ratio Ren, and the vertical axis indicates the welding current average value Iav. As is clear from the figure, when the EN ratio Ren changes, the welding current average value Iav varies significantly in a range of 100 to 140 A.
When forming beads into a desired shape on the workpiece, often the penetration depth and the area of the reinforcement portion are set to desired values. In general, the penetration depth changes with the welding current average value Iav, and the reinforcement area changes with the EN ratio Ren. However, as is clear from the figure, if the EN ratio Ren is changed in an attempt to set the reinforcement area, the welding current average value Iav also changes greatly at the same time, and the penetration depth also changes. That is, the penetration depth and the reinforcement area cannot be set independently to desired values. For this reason, numerous preparatory tests are necessary in order to set the bead shape to a desired value, and so considerable time has been required.